Pleural effusions are commonly encountered in patients with a variety of illnesses and often result in significant respiratory symptoms. The purpose of this study is to determine the effects of pleural effusions on respiratory system (RS) mechanics. Pleural effusions will be created in dogs to provide a model for examining changes in components of the RS. Dogs will be studied in head-up and supine body positions before and after infusion of a volume of saline equal to 50% of total lung capacity into the right pleural space. To examine changes in regional distribution of lung volume, single-breath oxygen maneuvers will be performed for the lungs overall, and for the ipsilateral (right) and contralateral (left) lungs separately, using a double-lumen endotracheal tube to isolate flows from the 2 lungs. The pleural pressure gradient will be determined indirectly, using serial esophageal pressure (Pes) measurements to assess local and overall effects of fluid in the pleural space. In a small series of dogs, the Pes-determined gradient will be compared to direct pleural space measurement. Deflation pressure-volume curves will be done for the lung and chest wall in the 4 study conditions (head-up and supine, pre and post effusion) and apparent changes in mechanics will be correlated with radiographic (positional) changes of thoracic structures. To assess responses of the RS to the load produced by effusion, effective elastance will be determined from RS pressures and tidal volume behavior in the four states studied. Emphasis will be given to quantitation of changes in the functional abilities of the diaphragm (DI) with changes in body position and with effusions. Transdiaphragmatic pressure (Pdi) will be measured as the difference in pressure above (Pes) and below (gastric pressure) the DI. Maximal Pdi (Pdi max) will be determined by supramaximal stimulation of the phrenic nerves. The ratio of Pdi/Pdi max is an index of the functional reserve of the DI. When related to inspiratory time/total respiratory cycle time, a time tension index will be obtained. A force-frequency curve for the DI will be generated by observing Pdi while stimulating the phrenic nerves at a series of frequencies. Change in body position and effusions will change the length of the DI and may impair function. With these experiments, a relatively complete description of RS function with effusion will be possible. Long term goals include: more detailed study of CW mechanics; EMG studies; and to gain an understanding of possible mechanisms for dyspnea and other symptoms in patients with effusions.